1. Field of the Invention
The present invention is directed to rotary motion linear actuators. More particularly, the present invention is directed to improved a rotary motion linear actuators which operate substantially without backlash or lost motion.
2. Brief Description of the Prior Art
Devices are well known in the prior art which convert rotary motion of a shaft to linear motion of a structure mounted on the shaft. Such devices are often called rotary motion linear actuators.
In one type of rotary motion linear actuator known in the prior art, the shaft has a helical (Archimedes) screw thread and a complementarily threaded ball nut is mounted on the shaft. A plurality of balls are disposed between the ball nut and the shaft. A suitable tube or the like is affixed to the ball nut to circulate the balls between the two ends of spiral grooves disposed in the ball nut. An object, such as a machine part, carriage or other body to be moved linearly, is fixedly attached to the ball nut, and is further affixed to a third body so that the object can not rotate together with the shaft. Therefore, when the shaft rotates, the ball nut moves linearly on the shaft and carries the object. Such rotary motion linear actuators having a ball nut on a ball screw threaded shaft are often used for actuating carriages or other linearly moving parts of machinery which require great precision of movement.
Another type of rotary motion linear actuator of the prior art utilizes conventional, unthreaded shafts. A housing is mounted on the shaft and contains a plurality of roller bearing type rollers. The roller bearings are disposed with their rotational axes at an angle to the axis of rotation of the shaft and are urged by a spring force or the like against the shaft. Rotary motion linear actuators of this type are often used where only light power loads are transmitted, or where the rotary motion linear actuator is required to stall or slip when an overload is encountered. Such applications include actuating mechanisms for power doors, windows of automobiles, carriages in computer housing cabinets and the like.
U.S. Pat. Nos. 3,272,021; 4,411,166; 3,425,284; 3,990,317; 4,246,802, and 4,224,831 disclose rotary motion linear actuators or like devices of the above-described second type. U.S. Pat. Nos. 3,589,202 and 3,443,443 disclose rotary motion linear actuators which utilize balls, rather than rollers, pressed against a shaft to convert rotary motion of the shaft into linear motion of the housing for the balls. Additional disclosures relating to rotary motion linear actuators, or to similar speed changing or power transmitting devices, are found in U.S. Pat. Nos. 4,253,342; 4,141,255; 3,186,250; 3,206,991; 2,642,756; 2,722,845, and 2,733,615.
A prior art rotary motion linear actuator which is available commercially in the United States under the trade mark ROH'LIX, is described in a brochure of its manufacturer, Zero-Max Industries Inc. of Minneapolis, Minn. This rotary motion linear actuator, like other prior art devices, also utilizes rollers pressed against a shaft with the rotational angle of the rollers disposed at a small acute angle to the shaft.
The above-noted and other prior art rotary motion linear actuators are not, however, without disadvantages. One problem shared by many prior art rotary motion linear actuators, particularly by the ones having a smooth (unthreaded) shaft, is the relatively small force which can be transmitted through the devices. The rotary motion linear actuators having both helically threaded ball screw type shaft and matching threaded ball nuts, on the other hand, are relatively expensive to manufacture.
Another problem shared by the prior art rotary motion linear actuators is backlash or lost motion. As it will be readily appreciated by those skilled in the art, backlash or lost motion is particularly troublesome in certain applications, such as carriages of machine tools, where great precision of movement is required. The problem of backlash was occasionally solved in the prior art by mounting two spring-connected, axially offset ball nuts on the same threaded shaft so that a predetermined pre-load was attained on the shaft. This solution, however, increased the bulk and cost of the actuator.
In light of the foregoing, there is still need in the prior art for linear motion rotary actuators having increased ability to transmit force, minimal lost motion and relatively low manufacturing costs. The present invention provides a rotary motion linear actuator having such improved characteristics.